Polypropylene resins have been widely used for fibrous applications because of their low cost and relatively good mechanical and spinning properties. Fibrous products of polypropylene resins, such as nonwoven fabrics require improvement in texture, lightness in weight and filtering property. A finer fiber is thus required.
In order to form finer fiber, spinning and winding must be carried out at higher speed than before in the spinning process. However high-speed spinning results in fiber breakage and thus lowers productivity.
Various materials have been designed to enhance the spinning property of polypropylene resins. For example, it is known that use of a polypropylene with a small molecular weight (large MFR) enhances the spinning property by lowering the melt viscosity of the resin. However the strength of the resulting fiber becomes low.
If spinning is carried out at high temperatures by raising the nozzle temperature, the melt viscosity can be lowered without need to reduce molecular weight. However the life of the nozzle is shortened due to gum formation, clogging and the like, and smoking increases.
As a means for improving spinning property without lowering melt viscosity (molecular weight) and changing the spinning temperature, it is known to decompose polypropylene with a high molecular weight using an organic peroxide, heat deterioration or the like and to use the resulting polypropylene with an adjusted molecular weight and narrow molecular weight distribution (for example, refer to JP-A-H8 (1996)-81593). However, this method has problems in that the peroxide causes color change and smoking occurs during spinning.
Regarding a solution to these problems it is reported that polypropylene having a narrow molecular weight distribution (e.g., Mw/Mn<3), for example, produced with a metallocene catalyst has an excellent high-speed spinning property (for example, refer to JP-T-H8 (1996)-510801, JP-A-H-101998)-183421 and JP-A-2000-136212).
It is also reported that spinning and other properties can be improved by removal of materials that cause smoking by cleansing with a solvent (for example, refer to JP-A-H11 (1999)-189914.)
Resins allowing higher speed spinning are still desired for enabling more finer fibers and enhanced productivity.
In view of the foregoing problems, an object of the invention is to provide a resin allowing high-speed spinning without changing the molecular weight of a resin, which affects the mechanical properties, or the spinning temperature and discharge rate, which affect productivity.